As is known in the art, fiber optic endoscopy is typically conducted by transmitting an image through an array of fibers often referred to as a fiber bundle. While successful for a variety of medical and non-medical applications, utilization of an array of fibers to form the image imposes constraints on the cost, diameter, and flexibility of the imaging device.
In an attempt to overcome these drawbacks, multiple approaches employing a single optical fiber have been proposed for miniature, flexible endoscopes. For example, one technique for confocal imaging with a single fiber has been implemented by utilizing the core of a single-mode fiber as both the source and the detection apertures. Also, miniature confocal microscope probes and endoscopes have been constructed by adding a mechanical micro-scanner at the tip of a single-mode fiber. Another single-fiber method for miniature endoscopy, termed spectral encoding, uses a broadband light source and a diffraction grating to spectrally encode reflectance across a transverse line within the sample as described in Tearney et al. Opt. Lett. 27: 412 (2002). A two-dimensional image is formed by slowly scanning this spectrally encoded line and a three-dimensional image may be obtained by placing the probe in the sample arm of an interferometer as described in Yelin et al. Opt. Left. 28: 2321 (2003). The core of the single-mode fiber acts as both the source and the detection apertures for all of these techniques.
As is also known, one important design parameter for single-fiber endoscopy is the modal profile of the optical fiber. Single-mode optical fibers enable high resolution imaging with small and flexible imaging probes, but suffer from relatively poor light throughput. Furthermore, the small core of the single-mode fiber acts similarly to a pinhole in free-space confocal microscopy, preventing the detection of out-of-focus light. For endoscopic applications, this optical sectioning may not be desirable since a large depth of field, large working distance, and wide field of view are typically preferred. For endoscopic microscopy applications, optical sectioning may be sacrificed for increased light throughput.
When illuminated by coherent sources, imaging via single-mode fibers also introduces so-called speckle noise, which significantly reduces the effective resolution and quality of the images. Replacing the single-mode fiber with a relatively large diameter multi-mode optical fiber enables higher optical throughput and decreases speckle. Unfortunately, utilization of a large diameter multi-mode fiber severely deteriorates the system's point-spread function and prevents the use of interferometry for high sensitivity and three-dimensional detection.
Recently, significant progress has been made developing high power fiber lasers utilizing double-clad (also called ‘dual-clad’) optical fibers. These fibers are unique in their ability to support single mode propagation through the core with multi-mode propagation through the inner cladding.